+
+
After completing this topic you should be able to :
•
State esters are formed by the condensation reaction between an alkanoic acid
and an alcohol.
•
State the ester link is formed by the reaction of a hydroxyl group with a carboxyl
group.
•
Name an ester given the names of the parent alkanol and alkanoic acid or from
shortened and full structural formulae.
•
Can draw shortened and full structural formulae for esters given the names of the
parent alkanol and alkanoic acid or the names of esters.
•
Describe an experiment where an ester is produced.
•
Give examples of the uses of esters.
When a CARBOXYLIC (ALKANOIC) ACID molecule reacts with an ALCOHOL
(ALKANOL) molecule a molecule called an ESTER forms.
The hydroxyl group of the alcohol reacts with the carboxyl group of the acid.
+
hydroxyl
group
+
carboxyl
group
ester link
water
molecule (carboxylate group)
During the reaction the H atom of the hydroxyl group and the OH of the carboxyl
group break off and join to form a water molecule.
The alcohol and acid molecules join to form an ester link (carboxylate group).
A reaction where a molecule of water is eliminated as two molecules join, is called a
CONDENSATION REACTION. Making an ester is a CONDENSATION REACTION.
The name of an ester comes from the alcohol and carboxylic acid from which it is
made.
The alcohol molecule gives
the ester its first name.
The alcohol name changes
to ALKYL.
The carboxylic acid
molecule gives the ester
its second name.
The acid name changes to
ALKANOATE.
propanol
+
ethanoic
acid
methanol
ethanol
propanol
methyl
ethyl
propyl
butanol
butyl
methanoic acid
ethanoic acid
propanoic acid
methanoate
ethanoate
propanoate
butanoic acid
butanoate
propyl
ethanoate
+
water
Making an ester is a condensation reaction.
The reaction is called ESTERIFICATION as the reaction produces an ester.
alcohol
+
carboxylic
acid
propanol
+
ethanoic
acid
ester
propyl
ethanoate
+
H
C C C O H
H H H
CH3CH2CH2OH
+
+
water
ester link
(carboxylate group)
H
O
+
water
+
water molecule
eliminated
H H H
+
C
H O
C
H
HOOCCH3
H H H
H
H
C C C
H H H
O
O
H
C
C
H
+
O
H
H
CH3CH2CH2OOCCH3 +
H2O
H
ethanol
+
H H
H
C C O H
H H
O
+
H H
CH3CH2OH
ethyl
methanoate
methanoic
acid
+
C
H
water
O
O
H
+
H
CH3CH2OOCH
+
H2O
H C C
H O
HOOCH
+
O
C
H H
A CONDENSATION REACTION is a reaction where molecules join with the
elimination of a small molecule (often water). Making an ester by reacting an
alcohol with a carboxylic acid is a condensation reaction.
Making an ester is also called ESTERIFICATION.
H
Esters can be recognised by the two oxygen atoms from the CARBOXYLATE GROUP
(COO or OOC) in the middle of the formula.
The formula / structure of an ester can be written with the alcohol part followed by the
acid part or vice-versa.
H
H
H
C
C
H
H
O
O
H
H
H
C
C
C
C
H
H
H
H
OR
H
H
H
H
O
C
C
C
C
H
H
H
O
H
H
C
C H
H
H
alcohol part
acid part
acid part
alcohol part
2 carbons
4 carbons
4 carbons
2 carbons
butanoate
butanoate
ethyl
ethyl
ethyl butanoate
The following examples shows how to name an ester from its shortened structural
formula or molecular formula.
SHORTENED STRUCTURAL
FORMULA
MOLECULAR FORMULA
CH3CH2OOCCH2CH2CH3
C2H5OOCC3H7
ethyl
ethyl
butanoate
butanoate
OR
CH3CH2 CH2 COOCH2CH3
butanoate
ethyl
C3H7 COOC2H5
butanoate
ethyl
To prepare an ester an alcohol (ethanol) has to be reacted with a carboxylic acid
(propanoic acid).
The rate of the reaction is slow at room temperature and the yield of ester is low. The
rate of the reaction can be increased by heating the reaction mixture and by using
concentrated sulphuric acid as a catalyst.
The presence of the concentrated sulphuric acid increases the yield of ester.
paper towel
soaked in cold
water
hot water
alcohol (ethanol) and
carboxylic acid
(propanoic acid) + a few
drops of concentrated
sulphuric acid
The mixture is heated for a
short time (10 mins).
After heating pour the contents of the test tube into
a solution of sodium hydrogencarbonate. This
reacts with any unused acid.
ester floats as an
oily layer on top of
the solution
sodium hydrogencarbonate
solution
The ester can be carefully smelled.
Esters have a pleasant fruity smell. Esters are found in fragrances giving flowers, fruit,
perfumes their characteristic smell. Esters are also used in flavourings.
Many esters have pleasant smells and flavours. They are found in nature in common
fruit flavours. They are found in coffee, tea, wine and beer.
Ester
Parent Alcohol
Parent Carboxylic
Acid
Flavouring /
Fragrance
propyl pentanoate
propan-1-ol
pentanoic acid
pineapple
ethyl butanoate
ethanol
butanoic acid
apple
octyl butanoate
octan-1-ol
ethanoic acid
orange
pentyl ethanoate
pentan-1-ol
ethanoic acid
banana
Esters are good solvents for varnishes, paints and adhesives. For example, butyl
ethanoate and ethyl ethanoate. These esters have a low boiling point and evaporate
easily i.e. they are volatile liquids.
Esters are found in medicines. They are used in local anaesthetics. For example,
benzocaine, cocaine, novocaine.
Aspirin, a common pain killer, is the ester called ethyl salicylate.